This invention relates to continuous transmission frequency modulated (CTFM) echo ranging systems, and more particularly to sonic ranging systems which produce a continuous target indication.
The echo ranging systems used in the past for locating underwater objects have operated on the principle of transmitting sound energy in the direction of the target and receiving a portion of the transmitted energy which is reflected back from the target. Since sound waves in ocean water travel at a substantially constant rate of about 1500 meters per second, the difference between the time of transmission of the sound energy and the time of reception of the relfected energy provides an accurate measure of the range to the target. Some of those ranging systems transmitted a series of discrete pulses separated in time from each other by an interval more than the transit time of the energy to and from a target at the maximum range of the equipment, and since the propagation rate of sound in ocean water is slightly less than a mile per second, the spaces between pulses must be several seconds. Such a system is therefore capable of searching an area only at very low speeds, although the operating range may be quite considerable.
However, from many applications the slow rate of search of the pulses sonic echo systems is a distinct disadvantage in maintaining contact with targets and, to overcome these disadvantages, frequency modulated systems have been devised. These systems, an example of which is provided in U.S. Pat. No. 2,977,568, employ a continuously radiating sinusoidal oscillator whose frequency is caused to vary cyclically between fixed limits. The reflected energy has the same frequency as the energy impinging on the target, and is combined with the output of the oscillator to produce a heterodyne or heat frequency. It will be apparent to those skilled in the art that the reflected energy will be delayed by the transit time to and from the target, and will therefore be of a different frequency than that of the oscillator, and that the difference .DELTA.f between the two frequencies will be a measure of the distance to the target. This is illustrated in FIG. 1.
The range resolution of a CTFM sonar is proportional to the number of band pass filters which are used to separate the different frequency echoes into range bins. For a given transmitted bandwidth, range resolution increases as more filters are added and the bandwidth of each is decreased. However, as more filters are added in an effort to achieve better range resolution, the linearity of the transmitted frequency sweep becomes more critical, because nonlinearity causes an error in the frequency difference between the transmitted signal and the echo signal. The result is that a target appears to wander back and forth through several range bins during the sweep time (T.sub.S). Conventional techniques, using a voltage ramp generator driving a voltage controlled oscillator, having rarely achieved 0.1% linearity, which is still inadequate for modern high-resolution sonars.
U.S. Pat. Nos. 3,530,404; 3,885,138; and 4,038,612 provide discussions and specific prior art examples of different approaches to highly linear FM sweep generation. In general, the techniques have relied on various forms corrective feedback, digital multiplication, and periodic phase coincidence comparison.
The manner in which the present invention achieves the desired signal generation will become apparent as this specification proceeds.